Inflammation represents a major independent risk factor for cardiovascular diseases. One of the prominent features associated with these diseases is the presence of abundant macrophages within the vascular lesions, which contribute to disease development by initiating and sustaining both inflammation and thrombosis and by causing subsequent rupture of the fibrous plaques. During the previous funding period, we studied migration of activated macrophages within an inflammatory environment and identified Mac-1, tPA, fibrin, PAI-1, and LRP as important players in this process. Based on these results, we hypothesize in this application that efficient macrophage migration within inflammatory environments, which functions critically in the resolution of acute inflammation and also during the progression of vascular lesions, depends on cooperation of three physiologically prominent systems (integrins, coagulation, and endocytosis). We propose that inflammation or vascular injury results in the formation of a fibrin-rich matrix. When activated, macrophages adhere to the provisional matrix fibrin that is complexed with the serine protease tPA. Subsequently, tPA is neutralized by its specific inhibitor PAI-1, which in turn enhances binding of the integrinprotease- inhibitor complex to the endocytic receptor LRP, and thus triggers a switch from cell adhesion to cell detachment and promotes receptor internalization. The internalized receptors are then recycled to the cell surface and the next cycle of adhesion, detachment and receptor internalization starts again. Such orderly transitions both spatially and temporally among the individual steps of cell migration lead to efficient macrophage migration. We plan to test this hypothesis using a panel of deficient mice, including Mac-1, fibrinogen, tPA, PAI-1, and LRP. We will also generate specific mutants of Mac-1, tPA, PAI-1, and LRP that are unable to interact with their respective partners and thus will not support the sequential assembly of the above protein complex or macrophage migration. The information obtained from this project will provide detailed mechanistic insights in macrophage migration within an inflammatory milieu and may help us better understand the molecular events that regulate either proper resolution of an acute inflammation under physiological conditions or the development of vascualr lesions within the vessel wall under pathological settings.